A seismic safety project to strengthen BART’s Transbay Tube will mean service delays over 14 months for some train riders late at night on weeknights.

Passengers taking the underwater tube between Oakland and San Francisco are subject to delays of up to 15 or 20 minutes after 10 p.m. on Tuesdays, Wednesdays and Thursdays, BART officials said. Those are the nights when crews will install giant steel plates along certain sections of the 3.6-mile-long tube to make it more resistant to earthquakes.

Begun last week, the project is expected to last for 14 months.

“We want people to know to leave at least 20 minutes earlier if they’re catching a late-night flight or have to be somewhere else at a given time,” said BART spokeswoman Luna Salaver. “We know this is an inconvenience for riders, but this project gives us a stronger tube.”

The unusual nature of the project makes passenger delays unavoidable, forcing BART to single-track trains through the tube during construction, officials said.

Crews must haul their heavy equipment into the tube each work night, and then haul it back out before train service begins the following morning.

“It’s like setting up and dismantling a construction site each night they work there,” Salaver said.

The transit system custom built a truck with a crane to haul the four-ton plates into the tube, where they will be bolted and welded in place.

The plates will shore up tube sections identified as needing reinforcements to withstand sideways movements during an earthquake.

For security reasons, BART does not identify which tube sections need bolstering.

About 193,700 passengers ride through the Transbay Tube each day.

The tube escaped damage in the 1989 Loma Prieta earthquake and reopened hours after the quake.